As light-transmitting fibers which have hitherto been developed, optical fibers using inorganic glass as the core and having excellent light-transmitting property are known, and the development of uses of the optical fibers is being advanced mainly in the field of light transmission by effectively utilizing such excellent light-transmission characteristics.
However, inorganic glass series optical fibers have disadvantages such as that the workability is poor, the flexibility is low, and the bending strength is so weak that they may be easily broken or deformed in the case of repeated bending. Accordingly, the development of optical fibers having no such disadvantages has been desired.
On the other hand, plastic optical fibers using a plastic such as polystyrene, polycarbonate, polymethyl methacrylate, etc., have features such as being high in flexibility and excellent in workability, and although the light-transmission characteristics thereof are inferior to those of inorganic glass optical fibers, the development of the uses of plastic optical fibers is being advanced mainly for the utilization thereof in the field of relatively short light-transmission distances, such as use for motor instruments or parts, use in office automation equipment, use between office automation equipment, etc.
In these plastic optical fibers, optical fibers using polymethyl methacrylate as the core and having excellent light-transmission characteristics of not more than 500 db/km, in particular not more than 200 db/km have been developed and since the appearance of such plastic optical fibers, the development of applications for them has been rapidly expanded.
However, the usable upper temperature limit of the plastic optical fibers using a polymethyl methacrylate resin as the core at ordinary use is as low as lower than 85.degree. C., since the thermal deformation temperature of polymethyl methacrylate itself is about 100.degree. C. Accordingly, in the case of using the optical fibers using a polymethyl methacrylate resin as the core, the atmospheric temperature must be lower than 100.degree. C., and such plastic optical fibers are unsatisfactory in heat resistance for use as optical fibers for communication among instruments in motor vehicles or optical fibers for use in office automation equipment. Therefore, the development of plastic optical fibers having heat resistance to temperatures of not lower than 120.degree. C., and more preferably not lower than 130.degree. C., and also having light transmission characteristics as good as those of polymethyl methacrylate series optical fibers has been keenly desired.
A plastic optical fiber having high heat resisting characteristics is disclosed in European Patent Publication (unexamined) No. 155,567. The optical fiber is a core-clad type optical fiber using a polyglutarimide series polymer, but has the following disadvantage. That is, a light emitting element which is advantageously used as a light source for plastic optical fibers has the main wavelengths of emitted light in the range of from 400 nm to 700 nm, but optical fibers using a polyglutarimide series polymer as the core are not so good in light transmission characteristics for light in the 400 nm to 700 nm range. For example, the light transmission loss of the aforesaid plastic optical fibers for light of 655 nm are 800 to 1,500 dB/km, which is larger than the light transmission loss (100 to 500 dB/km) of a polymethyl methacrylate series optical fibers for light of 655 nm. Furthermore, according to the inventors' investigations, it has been confirmed that plastic optical fibers having polyglutarimide as the core which have hitherto been developed have a property of rapidly increasing the light transmission loss for light of 400 to 700 nm over 5000 dB/km when the optical fibers are exposed to a high temperature atmosphere of higher than 100.degree. C., and hence the aforesaid plastic optical fibers are not yet fully sufficient for many uses as plastic optical fibers having high heat resistance and good light-transmission characteristics.